Failsafe multimode clutch assemblies for rotorcraft

ABSTRACT

A failsafe multimode clutch assembly is positioned in a powertrain of a rotorcraft. The clutch assembly includes a freewheeling unit having input and output races. The freewheeling unit has a driving mode in which torque applied to the input race is transferred to the output race and an overrunning mode in which torque applied to the output race is not transferred to the input race. A bypass assembly has an engaged position that couples the input and output races of the freewheeling unit. An actuator assembly must be energized to shift the bypass assembly from the engaged position to a disengaged position. In the disengaged position, the overrunning mode of the freewheeling unit is enabled such that the clutch assembly is configured for unidirectional torque transfer. In the engaged position, the overrunning mode of the freewheeling unit is disabled such that the clutch assembly is configured for bidirectional torque transfer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of co-pendingapplication Ser. No. 16/567,086, filed Sep. 11, 2019, which is acontinuation-in-part of application Ser. No. 16/274,520, filed Feb. 13,2019, which claims the benefit of provisional application No.62/801,621, filed Feb. 5, 2019, the entire contents of each are herebyincorporated by reference.

GOVERNMENT RIGHTS

This invention was made with Government support under Agreement No.W911W6-19-9-0002, awarded by the Army Contracting Command-RedstoneArsenal. The Government has certain rights in the invention.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to clutch assembliesoperable for use on rotorcraft and, in particular, to failsafe multimodeclutch assemblies operable to enable the selective use of secondaryengine power independent of or together with main engine power to drivethe main rotor, the tail rotor and/or the accessories of a rotorcraft.

BACKGROUND

Many rotorcraft are capable of taking off, hovering and landingvertically. One such rotorcraft is a helicopter, which has a main rotorthat provides lift and thrust to the aircraft. The main rotor not onlyenables hovering and vertical takeoff and landing, but also enablesforward, backward and lateral flight. These attributes make helicoptershighly versatile for use in congested, isolated or remote areas. It hasbeen found that the power demand of a rotorcraft can vary significantlybased upon the operation being performed. For example, low power demandexists during preflight operations, when power is only needed to operateaccessories such as generators, air pumps, oil pumps, hydraulic systemsand the like as well as to start the main engine. Certain rotorcraftutilize a dedicated auxiliary power unit to generate preflight accessorypower. During takeoff, hover, heavy lifts and/or high speed operations,rotorcraft experience high power demand. Certain rotorcraft utilizemultiple main engines or one main engine and a supplemental power unitto generate the required power for the main rotor during such high powerdemand flight operations. In conventional rotorcraft, the dedicatedauxiliary power unit is not operable to provide supplemental power tothe main rotor during high power demand flight operations. Accordingly,a need has arisen for improved rotorcraft systems that enable anauxiliary power unit to not only provide accessory power duringpreflight operations but also to operate as a supplemental power unit toprovide power to the main rotor during high power demand flightoperations.

SUMMARY

In a first aspect, the present disclosure is directed to a failsafemultimode clutch assembly for a rotorcraft. The clutch assembly includesa freewheeling unit having an input race and an output race. Thefreewheeling unit has a driving mode in which torque applied to theinput race is transferred to the output race and an overrunning mode inwhich torque applied to the output race is not transferred to the inputrace. A bypass assembly has an engaged position in which the bypassassembly couples the input and output races of the freewheeling unit anda disengaged position in which the bypass assembly does not couple theinput and output races of the freewheeling unit. An actuator assemblyhas a default configuration in which a pressurized lubricating oilprovides an engagement force that maintains the bypass assembly in theengaged position and an energized configuration in which a disengagementelement provides a disengagement force that overcomes the engagementforce and shifts the bypass assembly from the engaged position to thedisengaged position. In the disengaged position of the bypass assembly,the overrunning mode of the freewheeling unit is enabled such that theclutch assembly is configured for unidirectional torque transfer fromthe input race to the output race. In the engaged position of the bypassassembly, the overrunning mode of the freewheeling unit is disabled suchthat the clutch assembly is configured for bidirectional torque transferbetween the input and output races.

In some embodiments, the engagement force of the pressurized lubricatingoil may be configured to shift the bypass assembly from the disengagedposition to the engaged position when the disengagement force of thedisengagement element is not provided. In certain embodiments, theactuator assembly may include a liner and a piston that is slidablydisposed relative to the liner and is coupled to the bypass assembly. Insuch embodiments, the pressurized lubricating oil may act ondifferential areas of the piston to bias the bypass assembly toward theengaged position. Also, in such embodiments, the liner and the pistonmay define an oil chamber therebetween wherein the differential areasare annular differential areas of the piston forming opposite sides ofthe oil chamber. In some embodiments, the pressurized lubricating oilmay flow through the oil chamber. In certain embodiments, thedisengagement element may be a pressure switch such as a hydraulicswitch or a compressed air switch. In other embodiments, thedisengagement element may be an electric switch. In some embodiments,the default configuration of the actuator assembly may be an unenergizedstate of the disengagement element.

In a second aspect, the present disclosure is directed to a powertrainfor a rotorcraft. The powertrain has a main drive system including amain engine. The powertrain also has a secondary engine and a failsafemultimode clutch assembly that is positioned between the main drivesystem and the secondary engine. The clutch assembly includes afreewheeling unit having an input race coupled to the main drive systemand an output race coupled to the secondary engine. The freewheelingunit has a driving mode in which torque applied to the input race istransferred to the output race and an overrunning mode in which torqueapplied to the output race is not transferred to the input race. Abypass assembly has an engaged position in which the bypass assemblycouples the input and output races of the freewheeling unit and adisengaged position in which the bypass assembly does not couple theinput and output races of the freewheeling unit. An actuator assemblyhas a default configuration in which a pressurized lubricating oilprovides an engagement force that maintains the bypass assembly in theengaged position and an energized configuration in which a disengagementelement provides a disengagement force that overcomes the engagementforce and shifts the bypass assembly from the engaged position to thedisengaged position. In the disengaged position of the bypass assembly,the overrunning mode of the freewheeling unit is enabled such that theclutch assembly is configured for unidirectional torque transfer fromthe input race to the output race. In the engaged position of the bypassassembly, the overrunning mode of the freewheeling unit is disabled suchthat the clutch assembly is configured for bidirectional torque transferbetween the input and output races.

In some embodiments, the main engine may be a gas turbine engine and thesecondary engine may be a gas turbine engine. In other embodiments, themain engine may be a gas turbine engine and the secondary engine may bean electric motor. In certain embodiments, the secondary engine may beconfigured to generate between about 5 percent and about 20 percent ofthe power of the main engine or between about 10 percent and about 15percent of the power of the main engine.

In a third aspect, the present disclosure is directed to a rotorcraft.The rotorcraft includes a main rotor coupled to a main drive systemincluding a main engine. The rotorcraft also includes a secondary engineand a failsafe multimode clutch assembly that is positioned between themain drive system and the secondary engine. The clutch assembly includesa freewheeling unit having an input race coupled to the main drivesystem and an output race coupled to the secondary engine. Thefreewheeling unit has a driving mode in which torque applied to theinput race is transferred to the output race and an overrunning mode inwhich torque applied to the output race is not transferred to the inputrace. A bypass assembly has an engaged position in which the bypassassembly couples the input and output races of the freewheeling unit anda disengaged position in which the bypass assembly does not couple theinput and output races of the freewheeling unit. An actuator assemblyhas a default configuration in which a pressurized lubricating oilprovides an engagement force that maintains the bypass assembly in theengaged position and an energized configuration in which a disengagementelement provides a disengagement force that overcomes the engagementforce and shifts the bypass assembly from the engaged position to thedisengaged position. In the disengaged position of the bypass assembly,the overrunning mode of the freewheeling unit is enabled such that theclutch assembly is configured for unidirectional torque transfer fromthe input race to the output race. In the engaged position of the bypassassembly, the overrunning mode of the freewheeling unit is disabled suchthat the clutch assembly is configured for bidirectional torque transferbetween the input and output races.

In a preflight configuration of the rotorcraft, the bypass assembly isin the disengaged position, the main engine is not operating and thesecondary engine provides power to at least one rotorcraft accessory. Inan enhanced power configuration of the rotorcraft, the bypass assemblyis in the engaged position, the main engine provides power to the maindrive system and the secondary engine provides power to at least onerotorcraft accessory and to the main drive system through the clutchassembly. In a high efficiency configuration of the rotorcraft, thebypass assembly is in the engaged position, the secondary engine is instandby mode and the main engine provides power to the main drive systemand to at least one rotorcraft accessory through the clutch assembly. Inan enhanced autorotation configuration of the rotorcraft, the bypassassembly is in the engaged position, the main engine is not operatingand the secondary engine provides power to the main drive system throughthe clutch assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent disclosure, reference is now made to the detailed descriptionalong with the accompanying figures in which corresponding numerals inthe different figures refer to corresponding parts and in which:

FIGS. 1A-1C are schematic illustrations of a rotorcraft having amultimode clutch assembly in accordance with embodiments of the presentdisclosure;

FIGS. 2A-2E are block diagrams of a powertrain including a multimodeclutch assembly for a rotorcraft in various operating configurations inaccordance with embodiments of the present disclosure;

FIGS. 3A-3C are cross sectional views of a rotorcraft gearbox assemblyincluding a multimode clutch assembly in various operatingconfigurations in accordance with embodiments of the present disclosure;

FIGS. 4A-4B are cross sectional views of a rotorcraft gearbox assemblyincluding a multimode clutch assembly and depicting a lubricationcircuit in accordance with embodiments of the present disclosure;

FIGS. 5A-5B are cross sectional views of a rotorcraft gearbox assemblyincluding a multimode clutch assembly in various operatingconfigurations in accordance with embodiments of the present disclosure;

FIGS. 6A-6B are cross sectional views of a rotorcraft gearbox assemblyincluding a multimode clutch assembly in various operatingconfigurations in accordance with embodiments of the present disclosure;and

FIGS. 7A-7E are cross sectional views of a rotorcraft gearbox assemblyincluding a multimode clutch assembly and depicting various engagementstatus sensors in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

While the making and using of various embodiments of the presentdisclosure are discussed in detail below, it should be appreciated thatthe present disclosure provides many applicable inventive concepts,which can be embodied in a wide variety of specific contexts. Thespecific embodiments discussed herein are merely illustrative and do notdelimit the scope of the present disclosure. In the interest of clarity,all features of an actual implementation may not be described in thisspecification. It will of course be appreciated that in the developmentof any such actual embodiment, numerous implementation-specificdecisions must be made to achieve the developer's specific goals, suchas compliance with system-related and business-related constraints,which will vary from one implementation to another. Moreover, it will beappreciated that such a development effort might be complex andtime-consuming but would nevertheless be a routine undertaking for thoseof ordinary skill in the art having the benefit of this disclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, and the like described herein may be positioned inany desired orientation. Thus, the use of terms such as “above,”“below,” “upper,” “lower” or other like terms to describe a spatialrelationship between various components or to describe the spatialorientation of aspects of such components should be understood todescribe a relative relationship between the components or a spatialorientation of aspects of such components, respectively, as the devicesdescribed herein may be oriented in any desired direction. As usedherein, the term “coupled” may include direct or indirect coupling byany means, including by mere contact or by moving and/or non-movingmechanical connections.

Referring to FIGS. 1A-1C in the drawings, a rotorcraft in the form of ahelicopter is schematically illustrated and generally designated 10. Theprimary propulsion assembly of helicopter 10 is a main rotor assembly12. Main rotor assembly 12 includes a plurality of rotor blades 14extending radially outward from a main rotor hub 16. Main rotor assembly12 is coupled to a fuselage 18 and is rotatable relative thereto. Thepitch of rotor blades 14 can be collectively and/or cyclicallymanipulated to selectively control direction, thrust and lift ofhelicopter 10. A tailboom 20 is coupled to fuselage 18 and extends fromfuselage 18 in the aft direction. An anti-torque system 22 includes atail rotor assembly 24 coupled to an aft end of tailboom 20. Anti-torquesystem 22 controls the yaw of helicopter 10 by counteracting the torqueexerted on fuselage 18 by main rotor assembly 12. In the illustratedembodiment, helicopter 10 includes a vertical tail fin 26 that providestabilization to helicopter 10 during high speed forward flight. Inaddition, helicopter 10 includes wing members 28 that extend laterallyfrom fuselage 18 and wing members 30 that extend laterally from tailboom20. Wing members 28, 30 provide lift to helicopter 10 responsive to theforward airspeed of helicopter 10, thereby reducing the lift requirementon main rotor assembly 12 and increasing the top speed of helicopter 10

Main rotor assembly 12 and tail rotor assembly 24 receive torque androtational energy from a main engine 32. Main engine 32 is coupled to amain rotor gearbox 34 by suitable clutching and shafting. Main rotorgearbox 34 is coupled to main rotor assembly 12 by a mast 36 and iscoupled to tail rotor assembly 24 by tail rotor drive shaft 38. In theillustrated embodiment, a secondary engine 40 is coupled to tail rotordrive shaft 38 by a secondary gearbox 42. Together, main engine 32, mainrotor gearbox 34, tail rotor drive shaft 38, secondary engine 40 andsecondary gearbox 42 as well as various other shafts and gearboxescoupled therein may be considered as the powertrain of helicopter 10.

Secondary engine 40 is operable as an auxiliary power unit to providepreflight power to the accessories of helicopter 10 such as electricgenerators, air pumps, oil pumps, hydraulic systems and the like as wellas to provide the power required to start main engine 32. In addition,secondary engine 40 is operable to provide supplemental power to mainrotor assembly 12 that is additive with the power provided by mainengine 32 during, for example, high power demand conditions includingtakeoff, hover, heavy lifts and high speed flight operations. Secondaryengine 40 is also operable to provide emergency power to main rotorassembly 12. For example, in the event of a failure of main engine 32,secondary engine 40 is operable to provide emergency power to enhancethe autorotation and flare recovery maneuver of helicopter 10. Use ofsecondary engine 40 not only enhances the safety of helicopter 10 butalso increases the efficiency of helicopter 10. For example, having theextra power provided by secondary engine 40 during high power demandoperations allows main engine 32 to be downsized for more efficientsingle engine operations such as during cruise operations.

It should be appreciated that helicopter 10 is merely illustrative of avariety of aircraft that can implement the embodiments disclosed herein.Indeed, the multimode clutch assembly of the present disclosure may beimplemented on any rotorcraft. Other aircraft implementations caninclude hybrid aircraft, tiltwing aircraft, tiltrotor aircraft, quadtiltrotor aircraft, unmanned aircraft, gyrocopters, propeller-drivenairplanes, compound helicopters, drones and the like. As such, thoseskilled in the art will recognize that the multimode clutch assembly ofthe present disclosure can be integrated into a variety of aircraftconfigurations. It should be appreciated that even though aircraft areparticularly well-suited to implement the embodiments of the presentdisclosure, non-aircraft vehicles and devices can also implement theembodiments.

Referring to FIG. 2A in the drawings, a powertrain 100 of a rotorcraftis illustrated in a block diagram format. Powertrain 100 includes a mainengine 102 such as a turbo shaft engine capable of producing 2000 to4000 horsepower or more, depending upon the particular implementation.Main engine 102 is coupled to a freewheeling unit depicted as spragclutch 104 that acts as a one-way clutch enabling a driving mode inwhich torque from main engine 102 is coupled to main rotor gearbox 106when the rotating speed of the input race, on the main engine side ofsprag clutch 104, is matched with the rotating speed of the output race,on the main rotor gearbox side of sprag clutch 104. Importantly, spragclutch 104 has an overrunning mode in which main engine 102 is decoupledfrom main rotor gearbox 106 when the rotating speed of the input race isless than the rotating speed of the output race of sprag clutch 104.Operating sprag clutch 104 in the overrunning mode allows, for example,main rotor 108 of helicopter 10 to engage in autorotation in the eventof a failure of main engine 102.

In the illustrated embodiment, main rotor gearbox 106 is coupled tosprag clutch 104 via a suitable drive shaft. In addition, main rotorgearbox 106 is coupled to main rotor 108 by a suitable mast. Main rotorgearbox 106 includes a gearbox housing and a plurality of gears, such asplanetary gears, used to adjust the engine output speed to a suitablerotor speed so that main engine 102 and main rotor 108 may each rotateat optimum speed during flight operations of helicopter 10. Main rotorgearbox 106 is coupled to a tail rotor gearbox 110 via a suitable tailrotor drive shaft. Tail rotor gearbox 110 includes a gearbox housing anda plurality of gears that may adjust the main rotor gearbox output speedto a suitable rotational speed for operation of tail rotor 112. Mainengine 102, sprag clutch 104, main rotor gearbox 106 and tail rotorgearbox 110 as well as various shafts and gearing systems coupledtherewith may be considered the main drive system of powertrain 100.

Powertrain 100 includes a secondary engine 114 such as a turbo shaftengine or an electric motor capable of producing 200 to 400 horsepoweror more, depending upon the particular implementation. In theillustrated embodiment, secondary engine 114 may generate between about5 percent and about 20 percent or more of the horsepower of main engine102. In other embodiments, secondary engine 114 may generate betweenabout 10 percent and about 15 percent of the horsepower of main engine102. Secondary engine 114 is coupled to a secondary gearbox 116.Secondary engine 114 and secondary gearbox 116 as well as various shaftsand gearing systems coupled therewith may be considered the secondarydrive system of powertrain 100.

Referring additionally to FIG. 3A, secondary gearbox 116 includes afreewheeling unit depicted as sprag clutch 118 that acts as a one-wayclutch enabling a driving mode in which torque from secondary engine 114is coupled through sprag clutch 118 from an input race 120 to an outputrace 122. In the illustrated embodiment, output race 122 is coupled toan output gear 126 that provides power to accessories 124 such as one ormore generators, air pumps, oil pumps, hydraulic systems and the like.Sprag clutch 118 has an overrunning mode in which secondary engine 114is decoupled from torque transfer through sprag clutch 118 when therotating speed of input race 120 is less than the rotating speed ofoutput race 122. Operating sprag clutch 118 in the overrunning modeallows, for example, main engine 102 to drive accessories 124 whensecondary engine 114 is in standby mode or not operating, as discussedherein.

Secondary gearbox 116 includes a multimode clutch assembly 128 that iscoaxially aligned with sprag clutch 118 and secondary engine 114, in theillustrated embodiment. In other embodiments, multimode clutch assembly128 may operate on a separate axis than sprag clutch 118 and/orsecondary engine 114. Multimode clutch assembly 128 has a unidirectionaltorque transfer mode and a bidirectional torque transfer mode. In theillustrated embodiment, multimode clutch assembly 128 includes afreewheeling unit depicted as sprag clutch 130, a bypass assembly 132and an actuator assembly 134. Sprag clutch 130 has an input race 136that is coupled to main rotor gearbox 106 via the tail rotor drive shaftand one or more gears including input gear 138. Sprag clutch 130 has anoutput race 140 that is coupled to output race 122 of sprag clutch 118via shaft 122 a. Shaft 122 a has outer splines (not visible) that arecoupled to inner splines 140 a of output race 140. Likewise, shaft 122 ahas outer splines (not visible) that are coupled to inner splines (notvisible) of output race 122. Sprag clutch 130 may act as a one-wayclutch enabling a driving mode in which torque from the main drivesystem is coupled through sprag clutch 130 from input race 136 to outputrace 140. Sprag clutch 130 also has an overrunning mode in which themain drive system is decoupled from torque transfer with sprag clutch130 when the rotating speed of input race 136 is less than the rotatingspeed of output race 140 of sprag clutch 130. When sprag clutch 130 isacting as a one-way clutch, multimode clutch assembly 128 is in itsunidirectional torque transfer mode. In the unidirectional torquetransfer mode of multimode clutch assembly 128, torque can be drivenfrom the main drive system through secondary gearbox 116 but torquecannot be driven from secondary gearbox 116 to the main drive system ofpowertrain 100.

Referring additionally to FIG. 3C, the overrunning mode of multimodeclutch assembly 128 can be disabled by engaging bypass assembly 132 tocouple input race 136 and output race 140 of sprag clutch 130 tofunctionally form a connected shaft. In this configuration with bypassassembly 132 preventing sprag clutch 130 from operating in theoverrunning mode, multimode clutch assembly 128 is in its bidirectionaltorque transfer mode. In the bidirectional torque transfer mode ofmultimode clutch assembly 128, torque can be driven from the main drivesystem through secondary gearbox 116 and torque can be driven fromsecondary gearbox 116 to the main drive system of powertrain 100.

Multimode clutch assembly 128 is operated between the unidirectional andbidirectional torque transfer modes by shifting bypass assembly 132between its disengaged position (FIG. 3A) and its engaged position (FIG.3C). The operations of engaging and disengaging bypass assembly 132 maybe pilot controlled and/or may be automated by the flight controlcomputer of helicopter 10 and may be determined according to theoperating conditions of helicopter 10. In the illustrated embodiment,bypass assembly 132 is shifted between the engaged and disengagedpositions responsive to engagement and disengagement forces supplied byactuator assembly 134, which may be generated mechanically,electrically, hydraulically, pneumatically and/or combinations thereofor by other suitable actuation signaling means.

In the illustrated embodiment, actuator assembly 134 includes anactuator liner 142 that is fixed relative to the housing of secondarygearbox 116. A piston 144 is slidably and sealingly received withinactuator liner 142. In the illustrated embodiment, piston 144 is coupledto a piston extension depicted as an oil jet 146. In other embodiments,piston 144 and oil jet 146 may be integral or oil jet 146 may beomitted. Actuator assembly 134 also includes a bearing sled 148 that isslidably received about actuator liner 142 and that slidably receivespiston 144 therein. Bearing sled 148 and actuator liner 142 preferablyincluding an anti-rotation feature that prevents relative rotationtherebetween such as a tab and slot assembly wherein, for example, oneor more tabs of actuator liner 142 extend radially outwardly into slotsof bearing sled 148 or wherein one or more tabs of bearing sled 148extend radially inwardly into slots of actuator liner 142 (notpictured). In the illustrated embodiment, a mechanical biasing elementdepicted as wave spring 150 is positioned between a shoulder of piston144 and a shoulder bearing sled 148. A bearing assembly depicted as aball bearing set 152 couples bearing sled 148 with bypass assembly 132such that bypass assembly 132 translates with bearing sled 148 and isrotatable relative to bearing sled 148 as well as the other componentsof actuator assembly 134. In the illustrated embodiment, the inner raceof ball bearing set 152 has an anti-rotation coupling with bearing sled148. In addition, actuator assembly 134 includes an actuator 154 havinga cylinder 156 that is shiftable responsive to an electric signal, ahydraulic signal, a pneumatic signal or the like. When actuator 154 iselectrically signaled, actuator 154 may be referred to herein as anelectric switch. When actuator 154 is hydraulically or pneumaticallysignaled, actuator 154 may be referred to herein as a pressure switchand more precisely a hydraulic switch or a compressed air switch,respectively. Operation of cylinder 156 by actuator 154 causes piston144 to shift relative to actuator liner 142 between first and secondpositions. Shifting of piston 144 causes bypass assembly 132 to shiftbetween engaged and disengaged positions with sprag clutch 130. Morespecifically, bypass assembly 132 includes a shaft 132 a having outersplines (not visible) and a ring gear 132 b having outer splines (notvisible). The outer splines of shaft 132 a are in mesh with innersplines 140 a of output race 140 of sprag clutch 130 such that whenoutput race 140 is rotating, bypass coupling 132 also rotates. The outersplines of ring gear 132 b are selectively engaged with and disengagedfrom inner splines 136 a of input race 136 to operate multimode clutchassembly 128 between the unidirectional and bidirectional torquetransfer modes.

Returning to FIGS. 2A-2E, operating scenarios for helicopter 10 will nowbe described. In FIG. 2A, powertrain 100 is in a preflight configurationin which main engine 102 is not yet operating as indicated by the dashedlines between the components of the main drive system. As the main drivesystem is not turning, no torque is being applied to secondary gearbox116 from the main drive system as indicated by the dashed linetherebetween. Prior to starting secondary engine 114, an engagementstatus of multimode clutch assembly 128 should be checked. In theillustrated embodiment, an engagement status sensor includes threecircumferentially distributed inductive proximity sensors 158 (only onebeing visible in FIGS. 3A-3C) that are used to determine the engagementstatus of bypass assembly 132 by measuring the position of bearing sled148 relative to proximity sensors 158 by detecting the presence orabsence of the metal of bearing sled 148 adjacent to the faces ofproximity sensors 158. For example, as best seen in FIG. 3C, proximitysensors 158 detect the absence of bearing sled 148 relative theretoindicating bypass assembly 132 is in the engaged position. In addition,as best seen in FIGS. 3A and 3B, proximity sensors 158 detect thepresence of bearing sled 148 relative thereto indicating bypass assembly132 is not in the engaged position. In other embodiments, other numbersof proximity sensors 158 in other orientations may be used. In stillother embodiments, other types of engagement status sensors may be usedto determine the engagement status of bypass assembly 132, as will bediscussed herein. In addition to determining the engagement status ofbypass assembly 132 in preflight, the use of an engagement status sensoris also beneficial in determining, for example, a malfunction ofactuator assembly 134, breakage of wave spring 150, partial engagementor disengagement of bypass assembly 132, disengagement of bypassassembly 132 during flight, disengagement of bypass assembly 132 undertorque, engagement of bypass assembly 132 at a differential speedrelative to outer race 136 as well as other undesirable conditions.

Following the status check, if multimode clutch assembly 128 is not inthe unidirectional torque transfer mode with bypass assembly 132 in thedisengaged position, actuator 154 provides a suitable disengagementsignal (hydraulic, pneumatic, electric) to operate cylinder 156 andshift piston 144 to the position shown in FIG. 3A, thereby shiftingbypass assembly 132 to the disengaged position. It is noted that in thedisengaged position, contact between bypass assembly 132 and the housingof secondary gearbox 116 is prevented by bearing sled 148. Anotherstatus check may now be performed. Following the status check, ifmultimode clutch assembly 128 is in the unidirectional torque transfermode with bypass assembly 132 is in the disengaged position, secondaryengine 114 may be started such that secondary engine 114 provides torqueand rotational energy within the secondary drive system, as indicated bythe arrows between secondary engine 114, secondary gearbox 116 andaccessories 124, in FIG. 2A. More specifically, secondary engine 114 isdriving input race 120 of sprag clutch 118, which causes output race 122of sprag clutch 118 to drive output gear 126 which in turn providespower to accessories 124. It is noted that rotation of output race 122causes rotation of shaft 122 a which in turn causes rotation of outputrace 140 of sprag clutch 130, which is operation in its overrunningmode. In addition, rotation of shaft 122 a causes rotation bypassassembly 132 via inner splines 140 a. While operating in the preflightconfiguration, the pilot of helicopter 10 can proceed through thestartup procedure. Prior to starting main engine 102, the status ofmultimode clutch assembly 128 may be checked again using proximitysensors 158. This process step provides further assurance that bypassassembly 132 is secured in the disengaged position prior to startingmain engine 102.

Once main engine 102 is started, torque is delivered through the maindrive system as indicated by the arrows between the components withinthe main drive system, as best seen in FIG. 2B. In addition, the maindrive system may supply torque to secondary gearbox 116, as indicated bythe arrow therebetween. When power is applied to input race 136 of spragclutch 130 via input gear 138 from the main drive system such that inputrace 136 and output race 140 of sprag clutch 130 are turning together atthe same speed, multimode clutch assembly 128 may be operated from theunidirectional torque transfer mode to the bidirectional torque transfermode. Specifically, bypass assembly 132 can now be shifted from thedisengaged position to the engaged position responsive to pilot inputand/or operation of the flight control computer of helicopter 10. In theillustrated embodiment, actuator 154 provides a suitable engagementsignal (hydraulic, pneumatic, electric) to operate cylinder 156 andshift piston 144 to the position shown in FIG. 3B. In the illustratedconfiguration, the movement of piston 144 relative to actuator liner 142and bearing sled 148 has compressed wave spring 150 between piston 144and bearing sled 148 due to contact between the faces of the outersplines of ring gear 132 b and inner splines 136 a of input race 136.Wave spring 150 assists in overcoming such misalignment in the clockingof the outer splines of ring gear 132 b and inner splines 136 a of inputrace 136 by allowing full actuation of piston 144 while maintainingpressure between ring gear 132 b and input race 136 so that when bypassassembly 132 and input race 136 start to rotate relative to each other,the outer splines of ring gear 132 b will mesh with inner splines 136 aof input race 136, thereby shifting bypass assembly 132 to the engagedposition and multimode clutch assembly 128 to the bidirectional torquetransfer mode, as best seen in FIG. 3C.

If the outer splines of ring gear 132 b and inner splines 136 a of inputrace 136 are aligned prior to operating cylinder 156, bypass assembly132 may be shifted directly from the disengaged position (FIG. 3A) tothe engaged position (FIG. 3C) without compressing spring 150 or beingin the intermediate position depicted in FIG. 3B. In the bidirectionaltorque transfer mode of multimode clutch assembly 128, when input race136 of sprag clutch 130 is driven by the main drive system, bypassassembly 132 and output race 140 rotate therewith. In addition, whenoutput race 140 of sprag clutch 130 is driven by secondary engine 114,bypass assembly 132 and input race 136 rotate therewith to supply powerto main drive system, thereby bypassing the overrunning mode of spragclutch 130 such that multimode clutch assembly 128 operates with thefunctionality of a connected shaft. Actuator assembly 134 preferably hasa suitable locking mechanism to maintain bypass assembly 132 in theengaged position until it is desired to shift bypass assembly 132 to thedisengaged position.

In the engaged position, bypass assembly 132 couples input race 136 withoutput race 140 such that multimode clutch assembly 128 is in thebidirectional torque transfer mode. In this configuration, secondaryengine 114 may be operated in standby mode or powered down as indicatedby the dashed line between secondary engine 114 and secondary gearbox116 in FIG. 2C, such that main engine 102 is driving not only the maindrive system but also accessories 124, as indicated by the arrows tosecondary gearbox 116 and accessories 124. This configuration ofpowertrain 100 may be referred to as a high efficiency configuration. Inaddition, secondary engine 114 may be operated to provide supplementalpower to the main drive system as indicated by the arrow betweensecondary gearbox 116 and the tail rotor drive shaft in FIG. 2D. Thisconfiguration of powertrain 100 may be referred to as an enhanced powerconfiguration.

Continuing with the operating scenarios of helicopter 10, once multimodeclutch assembly 128 is in the bidirectional torque transfer mode,helicopter 10 is ready for takeoff. Assuming a high power demand takeoffand/or hover, powertrain 100 is preferably in the enhanced powerconfiguration of FIG. 2D for takeoff. Once helicopter 10 has completedthe takeoff and is flying at a standard speed cruise, it may bedesirable to place secondary engine 114 in standby mode such as idleoperations or even shut secondary engine 114 down to operate helicopter10 in the high efficiency configuration depicted in FIG. 2C. In thisconfiguration, secondary engine 114 provide no power as indicated by thedashed line between secondary engine 114 and secondary gearbox 116 withtorque and rotational energy being provided by main engine 102 throughthe main drive system to secondary gearbox 116 and accessories 124. Morespecifically, power from the main drive system is transferred throughmultimode clutch assembly 128 to output gear 126 by input race 136 andoutput race 140 that are coupled together by bypass assembly 132 then byshaft 122 a and output race 122 of sprag clutch 118. Rotational energyis not sent to input race 120, as sprag clutch 118 is operating in itsoverrunning mode. Thus, in addition to powering main rotor 108 and tailrotor 112, in the high efficiency configuration of powertrain 100, mainengine 102 also powers accessories 124.

It should be noted that multimode clutch assembly 128 is preferablymaintained in its bidirectional torque transfer mode during all flightoperations. For example, having multimode clutch assembly 128 in itsbidirectional torque transfer mode is a safety feature of helicopter 10in the event of a failure in main engine 102 during flight, as indicatedby the dashed lines between main engine 102 and sprag clutch 104 in FIG.2E. In this case, an autorotation maneuver may be performed in which thedescent of helicopter 10 creates an aerodynamic force on main rotor 108as air moves up through main rotor 108 generating rotational inertia.Upon final approach during the autorotation landing, helicopter 10performs a flare recovery maneuver in which the kinetic energy of mainrotor 108 is converted into lift using aft cyclic control. Both theautorotation maneuver and the flare recovery maneuver are enhanced byoperating secondary engine 114 and sending power through secondarygearbox 116 to the main drive system, as indicated by the arrowtherebetween, and more particularly by sending power to main rotor 108as indicated by the arrows leading thereto. It is noted that rotationalenergy is also sent to sprag clutch 104, which is operating in itsoverrunning mode while main engine 102 is not operating. Thisconfiguration may be referred to as the enhanced autorotationconfiguration of powertrain 100 in which main engine 102 is notoperating but secondary engine 114 is providing power to main rotor 108through multimode clutch assembly 128, which is in the bidirectionaltorque transfer mode.

Continuing with the operating scenarios of helicopter 10, after aconventional landing, when it is desired to operate multimode clutchassembly 128 from the bidirectional to the unidirectional torquetransfer mode, main engine 102 continues to provide torque androtational energy to input race 136, which in turn drives output race140 of sprag clutch 130. Actuator 154 then provides a suitabledisengagement signal (hydraulic, pneumatic, electric) to operatecylinder 156 and shift piston 144 to the position shown in FIG. 3A suchthat the outer splines of ring gear 132 b shift out of mesh with innersplines 136 a of input race 136, thereby shifting bypass assembly 132 tothe disengaged position. Actuator assembly 134 preferably has a suitablelocking mechanism to maintain bypass assembly 132 in the disengagedposition until it is desired to shift bypass assembly 132 to the engagedposition.

Referring next to FIGS. 4A-4B, the lubrication strategy for secondarygearbox 116 will now be described. Secondary gearbox 116 includes alubrication circuit in which pressurized lubricating oil is depicted asheavy dashed lines 200. The lubrication circuit includes an oil pump(not pictured) that pressurizes and routes lubricating oil to secondarygearbox 116 and in particular to supply port 202. Pressurizedlubricating oil 200 is then routed to an annular passageway 204 definedbetween the housing of secondary gearbox 116 and actuator liner 142 by apair of seals depicted as O-rings. Actuator liner 142 includes one ormore passageways 206 that route pressurized lubricating oil 200 to anannular oil chamber 208 defined between actuator liner 142 and piston144 by a pair of seals depicted as O-rings 144 a, 144 b. Pressurizedlubricating oil 200 then enters the interior of piston 144 via one ormore ports 210 that are in fluid communication with annular oil chamber208. From piston 144, pressurized lubricating oil 200 flows into oil jet146 that includes a plurality of nozzles 146 a, 146 b, 146 c, 146 d, 146e, 146 f. A filter or debris screen (not pictured) may be positionedwithin piston 144 to prevent any solids within pressurized lubricatingoil 200 from entering oil jet 146 and the plugging nozzles.

Each of the nozzles directs pressurized lubricating oil 200 into aspecific region within shaft 122 a defined between adjacent oil dams.More specifically, one or more nozzles 146 a direct pressurizedlubricating oil 200 into region 212, one or more nozzles 146 b directpressurized lubricating oil 200 into region 214, one or more nozzles 146c direct pressurized lubricating oil 200 into region 216, one or morenozzles 146 d direct pressurized lubricating oil 200 into region 218,one or more nozzles 146 e direct pressurized lubricating oil 200 intoregion 220 and one or more nozzles 146 f direct pressurized lubricatingoil 200 into region 222. The centrifugal force generated by rotation ofshaft 122 a during operation of helicopter 10 aids in oil flow from theinterior of shaft 122 a to the desired locations within secondarygearbox 116. For example, pressurized lubricating oil 200 from region212 flows to ball bearing set 152 for lubrication thereof. Similarly,pressurized lubricating oil 200 from region 216 flows to sprag clutch130 to provide lubrication for the sprag elements 130 a between inputrace 136 and output race 140 as well as for clutch bearing sets 130 b,130 c. Oil dams within sprag clutch 130 keep sprag elements 130 asubmerged in pressurized lubricating oil 200. The oil dams may alsoinclude metering orifices that route pressurized lubricating oil 200 toclutch bearing sets 130 b, 130 c. Likewise, pressurized lubricating oil200 from region 222 flows to sprag clutch 118 to provide lubrication forthe sprag elements 118 a between input race 120 and output race 122 aswell as for clutch bearing sets 118 b, 118 c. Oil dams within spragclutch 118 keep sprag elements 118 a submerged in pressurizedlubricating oil 200. The oil dams may also include metering orificesthat route pressurized lubricating oil 200 to clutch bearing sets 118 b,118 c. Importantly, lubrication circuit integrity is maintained whenbypass assembly 132 is shifted between the engaged and disengagedpositions as the oil inlet to annular oil chamber 208 remains betweenO-ring 144 a, 144 b as piston 144 shifts within actuator liner 142between the disengaged position of bypass assembly 132 (FIG. 4A) and theengaged position of bypass assembly 132 (FIG. 4B).

As discussed herein, multimode clutch assembly 128 is preferablymaintained in its bidirectional torque transfer mode during all flightoperations. This is achieved in the embodiment depicted in FIGS. 5A-5Busing a mechanical biasing element that maintains bypass assembly 132 inthe engaged position unless a disengagement force sufficient to overcomethe engagement force of the mechanical biasing element is applied.Specifically, a secondary gearbox 300 includes sprag clutch 118 havinginput race 120 and output race 122 which is coupled to output gear 126that provides power to accessories 124. Secondary gearbox 300 alsoincludes a multimode clutch assembly 128 that is coaxially aligned withsprag clutch 118. Multimode clutch assembly 128 has a unidirectionaltorque transfer mode and a bidirectional torque transfer mode. Multimodeclutch assembly 128 includes sprag clutch 130, bypass assembly 132 andan actuator assembly 302. Sprag clutch 130 includes input race 136 thatis coupled to main rotor gearbox 106 via the tail rotor drive shaft andone or more gears including input gear 138. Sprag clutch 130 includesoutput race 140 that is coupled to output race 122 of sprag clutch 118via shaft 122 a. Sprag clutch 130 may act as a one-way clutch enabling adriving mode in which torque from the main drive system is coupledthrough sprag clutch 130 from input race 136 to output race 140. Spragclutch 130 also has an overrunning mode in which the main drive systemis decoupled from torque transfer with sprag clutch 130 when therotating speed of input race 136 is less than the rotating speed ofoutput race 140 of sprag clutch 130. When sprag clutch 130 is acting asa one-way clutch, multimode clutch assembly 128 is in its unidirectionaltorque transfer mode. In the unidirectional torque transfer mode ofmultimode clutch assembly 128, torque can be driven from the main drivesystem through secondary gearbox 300 but torque cannot be driven fromsecondary gearbox 300 to the main drive system of powertrain 100.

The overrunning mode of multimode clutch assembly 128 can be disabled byengaging bypass assembly 132 to couple input race 136 and output race140 of sprag clutch 130 to functionally form a connected shaft. In thisconfiguration with bypass assembly 132 preventing sprag clutch 130 fromoperating in the overrunning mode, multimode clutch assembly 128 is inits bidirectional torque transfer mode. In the bidirectional torquetransfer mode of multimode clutch assembly 128, torque can be drivenfrom the main drive system through secondary gearbox 300 and torque canbe driven from secondary gearbox 300 to the main drive system ofpowertrain 100.

Multimode clutch assembly 128 is operated between the unidirectional andbidirectional torque transfer modes by shifting bypass assembly 132between its disengaged position (FIG. 5A) and its engaged position (FIG.5B). The operations of engaging and disengaging bypass assembly 132 maybe pilot controlled and/or may be automated by the flight controlcomputer of helicopter 10 and may be determined according to theoperating conditions of helicopter 10. In the illustrated embodiment,bypass assembly 132 is shifted between the engaged and disengagedpositions responsive to engagement and disengagement forces supplied byactuator assembly 302.

Actuator assembly 302 includes an actuator liner 304 that is fixedrelative to the housing of secondary gearbox 300. A piston 306 isslidably and sealingly received within actuator liner 304. In theillustrated embodiment, piston 306 is coupled to a piston extensiondepicted as oil jet 146. Actuator assembly 302 also includes a bearingsled 308 that is slidably received about actuator liner 304. Bearingsled 308 is coupled to piston 306 to prevent relative translationtherebetween and thus, may be considered part of piston 306. In theillustrated embodiment, a mechanical biasing element depicted as wavespring 310 is positioned between a shoulder of actuator liner 304 and anend of bearing sled 308. A bearing assembly depicted as ball bearing set152 couples bearing sled 308 with bypass assembly 132 such that bypassassembly 132 is rotatable relative to bearing sled 308 as well as theother components of actuator assembly 302. In addition, actuatorassembly 302 includes an actuator 312 having a cylinder 314 that isshiftable responsive to an electric signal, a hydraulic signal, apneumatic signal or the like. In the illustrated embodiment, actuatorassembly 302 has an energized configuration in which cylinder 314 isretracted, as depicted in FIG. 5A, and an unenergized or defaultconfiguration in which cylinder 314 is released, as depicted in FIG. 5B.

When actuator 312 is not activated, the biasing force generated by wavespring 310 acts on bearing sled 308 and serves as an engagement force toshift bypass assembly 132 from the disengaged position (FIG. 5A) to theengaged position (FIG. 5B). In addition, once bypass assembly 132 is inthe engaged position, the biasing force generated by wave spring 310continues to act on bearing sled 308 to maintain the engagement force onbypass assembly 132, thereby preventing bypass assembly 132 fromshifting out of the engaged position. The use of actuator assembly 302with wave spring 310 makes multimode clutch assembly 128 a mechanicallyfailsafe multimode clutch assembly that remains in the bidirectionaltorque transfer mode even if a failure occurs in a related electric,hydraulic and/or pneumatic system. When helicopter 10 has landed and itis desired to shift bypass assembly 132 from the engaged position (FIG.5B) to the disengaged position (FIG. 5A), actuator 312 is energized withthe appropriate electric signal, hydraulic signal, pneumatic signal orthe like to generate a disengagement force that overcomes the engagementforce of wave spring 310 causing cylinder 314 to shift piston 306relative to actuator liner 304 which compresses wave spring 310 betweenactuator liner 304 and bearing sled 308 and shifts bypass assembly 132to the disengaged position. In the illustrated embodiment, actuator 312must remain energized to overcome the engagement force of wave spring310. Actuator assembly 302 may have a suitable locking mechanism tosecure bypass assembly 132 in the disengaged position until it isdesired to shift bypass assembly 132 to the engaged position, in whichcase, actuator assembly 302 may be deenergized after the lockingmechanism has been deployed.

Alternatively or additionally, actuator 312 may be used to provide atleast a portion of the engagement force to shift bypass assembly 132from the disengaged position (FIG. 5A) to the engaged position (FIG.5B). For example, actuator 312 may be energized with the appropriateelectric signal, hydraulic signal, pneumatic signal or the like togenerate at least a portion of the engagement force that together withthe biasing force generated by wave spring 310 shifts bypass assembly132 from the disengaged position to the engaged position. In thisembodiment, once bypass assembly 132 is in the engaged position,actuator 312 may be unenergized as the biasing force generated by wavespring 310 continues to act on bearing sled 308 to maintain theengagement force on bypass assembly 132, thereby preventing bypassassembly 132 from shifting out of the engaged position.

FIGS. 6A-6B depict another embodiment of a secondary gearbox thatincludes a failsafe multimode clutch assembly. In this embodiment, apressurized fluid maintains bypass assembly 132 in the engaged positionunless a disengagement force sufficient to overcome the engagement forceof the pressurized fluid is applied. Specifically, secondary gearbox 400includes sprag clutch 118 having input race 120 and output race 122which is coupled to output gear 126 that provides power to accessories124. Secondary gearbox 400 also includes a multimode clutch assembly 128that is coaxially aligned with sprag clutch 118. Multimode clutchassembly 128 has a unidirectional torque transfer mode and abidirectional torque transfer mode. Multimode clutch assembly 128includes sprag clutch 130, bypass assembly 132 and an actuator assembly402. Sprag clutch 130 includes input race 136 that is coupled to mainrotor gearbox 106 via the tail rotor drive shaft and one or more gearsincluding input gear 138. Sprag clutch 130 includes output race 140 thatis coupled to output race 122 of sprag clutch 118 via shaft 122 a. Spragclutch 130 may act as a one-way clutch enabling a driving mode in whichtorque from the main drive system is coupled through sprag clutch 130from input race 136 to output race 140. Sprag clutch 130 also has anoverrunning mode in which the main drive system is decoupled from torquetransfer with sprag clutch 130 when the rotating speed of input race 136is less than the rotating speed of output race 140 of sprag clutch 130.When sprag clutch 130 is acting as a one-way clutch, multimode clutchassembly 128 is in its unidirectional torque transfer mode. In theunidirectional torque transfer mode of multimode clutch assembly 128,torque can be driven from the main drive system through secondarygearbox 400 but torque cannot be driven from secondary gearbox 400 tothe main drive system of powertrain 100.

The overrunning mode of multimode clutch assembly 128 can be disabled byengaging bypass assembly 132 to couple input race 136 and output race140 of sprag clutch 130 to functionally form a connected shaft. In thisconfiguration with bypass assembly 132 preventing sprag clutch 130 fromoperating in the overrunning mode, multimode clutch assembly 128 is inits bidirectional torque transfer mode. In the bidirectional torquetransfer mode of multimode clutch assembly 128, torque can be drivenfrom the main drive system through secondary gearbox 400 and torque canbe driven from secondary gearbox 400 to the main drive system ofpowertrain 100.

Multimode clutch assembly 128 is operated between the unidirectional andbidirectional torque transfer modes by shifting bypass assembly 132between its disengaged position (FIG. 6A) and its engaged position (FIG.6B). The operations of engaging and disengaging bypass assembly 132 maybe pilot controlled and/or may be automated by the flight controlcomputer of helicopter 10 and may be determined according to theoperating conditions of helicopter 10. In the illustrated embodiment,bypass assembly 132 is shifted between the engaged and disengagedpositions responsive to engagement and disengagement forces supplied byactuator assembly 402.

Actuator assembly 402 includes an actuator liner 404 that is fixedrelative to the housing of secondary gearbox 400. A piston 406 isslidably and sealingly received within actuator liner 404. In theillustrated embodiment, piston 406 is coupled to a piston extensiondepicted as oil jet 146. Actuator assembly 402 also includes a bearingsled 408 that is slidably received about actuator liner 404 and thatslidably receives piston 406 therein. In the illustrated embodiment, amechanical biasing element depicted as wave spring 410 is positionedbetween a shoulder of piston 406 and a shoulder of bearing sled 408.Wave spring 410 operates in a manner similar to wave spring 150discussed herein to assist in overcoming any misalignment in theclocking between splines of bypass assembly 132 and input race 136during engagement operations. A bearing assembly depicted as ballbearing set 152 couples bearing sled 408 with bypass assembly 132 suchthat bypass assembly 132 is rotatable relative to bearing sled 408 aswell as the other components of actuator assembly 402. In addition,actuator assembly 402 includes an actuator 412 having a cylinder 414that is shiftable responsive to an electric signal, a hydraulic signal,a pneumatic signal or the like. In the illustrated embodiment, actuatorassembly 402 has an energized configuration in which cylinder 414 isretracted, as depicted in FIG. 6A, and an unenergized or defaultconfiguration in which cylinder 414 is released, as depicted in FIG. 6B.

Similar to the lubrication circuit described herein with reference toFIGS. 4A-4B, secondary gearbox 400 includes a lubrication circuit thatnot only provides pressurized lubricating oil to various componentswithin secondary gearbox 400 but also provides a pressure source forfailsafe operations of bypass assembly 132. In particular, thelubrication circuit of secondary gearbox 400 includes an oil pump (notpictured) that pressurizes and routes lubricating oil 416 to supply port418. Pressurized lubricating oil 416 is then routed to an annularpassageway 420 defined between the housing of secondary gearbox 400 andactuator liner 404 by a pair of seals depicted as O-rings. Actuatorliner 404 includes one or more passageways 422 that route pressurizedlubricating oil 416 to an annular oil chamber 424 defined betweenactuator liner 404 and piston 406 by a pair of seals depicted as O-rings406 a, 406 b. While not illustrated, pressurized lubricating oil 416then enters the interior of piston 406 for distribution to variouscomponents via nozzles of oil jet 146, as discussed herein. In theillustrated embodiment, annular oil chamber 424 and O-rings 406 a, 406 bdefined a differential pressure chamber as the annular area defined byO-ring 406 b is larger than the annular area defined by O-ring 406 asuch that when pressurized lubricating oil 416 flows through annular oilchamber 424, a biasing force is generated that acts on piston 406 andserves as an engagement force to shift bypass assembly 132 from thedisengaged position (FIG. 6A) to the engaged position (FIG. 6B). Inaddition, once bypass assembly 132 is in the engaged position, thebiasing force generated by pressurized lubricating oil 416 in annularoil chamber 424 continues to act on piston 406 to maintain theengagement force on bypass assembly 132, thereby preventing bypassassembly 132 from shifting out of the engaged position.

The use of actuator assembly 402 with pressurized lubricating oil 416 inannular oil chamber 424 makes multimode clutch assembly 128 ahydraulically failsafe multimode clutch assembly that remains in thebidirectional torque transfer mode even if a failure occurs in a relatedelectric, hydraulic and/or pneumatic system. When helicopter 10 haslanded and it is desired to shift bypass assembly 132 from the engagedposition (FIG. 6B) to the disengaged position (FIG. 6A), actuator 412 isenergized with the appropriate electric signal, hydraulic signal,pneumatic signal or the like to generate a disengagement force thatovercomes the engagement force of pressurized lubricating oil 416 inannular oil chamber 424 causing cylinder 414 to shift piston 406relative to actuator liner 404 which in turn shifts bypass assembly 132to the disengaged position. In the illustrated embodiment, actuator 412must remain energized to overcome the engagement force of pressurizedlubricating oil 416 in annular oil chamber 424. Actuator assembly 402may have a suitable locking mechanism to secure bypass assembly 132 inthe disengaged position until it is desired to shift bypass assembly 132to the engaged position, in which case, actuator assembly 402 may bedeenergized after the locking mechanism has been deployed.

Alternatively or additionally, actuator 412 may be used to provide atleast a portion of the engagement force to shift bypass assembly 132from the disengaged position (FIG. 6A) to the engaged position (FIG.6B). For example, actuator 412 may be energized with the appropriateelectric signal, hydraulic signal, pneumatic signal or the like togenerate at least a portion of the engagement force that together withthe biasing force generated by pressurized lubricating oil 416 inannular oil chamber 424 shifts bypass assembly 132 from the disengagedposition to the engaged position. In this embodiment, once bypassassembly 132 is in the engaged position, actuator 412 may be unenergizedas the biasing force generated by pressurized lubricating oil 416 inannular oil chamber 424 continues to act on piston 406 to maintain theengagement force on bypass assembly 132, thereby preventing bypassassembly 132 from shifting out of the engaged position.

As discussed herein, maintaining bypass assembly 132 in the engagedposition during all flight operations is an important safety feature ofthe present helicopter to ensure, for example, that the secondary enginecan provide power to the main rotor in the event of a main enginefailure. Depending upon the specific configuration of the multimodeclutch assembly, a variety of engagement status sensors may be used tomonitor the engagement status of the multimode clutch assembly. In oneexample, FIG. 7A depicts secondary gearbox 116 with bypass assembly 132in the engaged position. In the illustrated embodiment, multimode clutchassembly 128 includes one or more proximity sensors depicted as one ormore load cells 500. Load cells 500 may be coupled to an end of shaft122 a such that translation of bypass assembly 132 brings an end ofshaft 132 a into contact with load cells 500 when bypass assembly 132 isin the engaged position. In one example, load cells 500 may becompression load cells having strain gauges that provide an electricalsignal to indicate the presence or absence of a load and/or an absoluteload between a no-load condition and a full-capacity load. Suchcompression load cells may also be referred to herein as strain sensors.In operation, a no-load reading by load cells 500 indicates bypassassembly 132 is not in the engaged position while a load readingindicates bypass assembly 132 is in the engaged position, therebyproviding the engagement status of bypass assembly 132. Alternatively, aload reading below a predetermined threshold by load cells 500 indicatesbypass assembly 132 is not in the engaged position while a load readingabove a predetermined threshold indicates bypass assembly 132 is in theengaged position, thereby providing the engagement status of bypassassembly 132.

In another example, FIG. 7B depicts secondary gearbox 116 with bypassassembly 132 in the engaged position. In the illustrated embodiment,multimode clutch assembly 128 includes one or more engagement statussensors depicted as one or more tooth passage frequency sensors 502. Forexample, tooth passage frequency sensors 502 could be variablereluctance sensors, monopole sensors, hall-effect sensors, opticalsensors or the like. In the illustrated embodiment, bypass assembly 132includes two ring gears 132 b, 132 c. The number of splines on ring gear132 b is different from the number of teeth on ring gear 132 c such asin a ratio of 2 or 3 to 1 or in a ratio of 1 to 2 or 3. When bypassassembly 132 is in the engaged position, tooth passage frequency sensors502 are aligned with rotating ring gear 132 c such that the alternatingpresence and absence of the passing gear teeth has a first frequency.When bypass assembly 132 is in the disengaged position, tooth passagefrequency sensors 502 are aligned with rotating ring gear 132 b whichhas a different number of splines than the number of teeth of ring gear132 c such that the alternating presence and absence of the passingsplines has a second frequency. The frequency detected by tooth passagefrequency sensors 502 is different for ring gear 132 b versus ring gear132 c such that the change in frequency and/or the absolute frequencyprovides the engagement status of bypass assembly 132.

When tooth passage frequency sensors 502 are variable reluctancesensors, for example, the alternating presence and absence of thepassing gear teeth vary the reluctance of a magnetic field, whichdynamically changes the magnetic field strength. This changing magneticfield strength induces a current into a coil winding which is attachedto the output terminals such that the variable reluctance sensorsprovide a frequency output. Alternatively or additionally, tooth passagefrequency sensors 502 may be used to detect a change in the annularspeed of bypass assembly 132 in the engaged position versus thedisengage position, even in embodiments having the same number of teethon both ring gears 132 b, 132 c. In this implementation, tooth passagefrequency sensors 502 provide a first frequency reading when bypassassembly 132 is in the engaged position and a second frequency reading,based upon a lower or a higher annular speed of bypass assembly 132depending upon the status of secondary engine 114, when bypass assembly132 is in the disengaged position, thereby providing the engagementstatus of bypass assembly 132.

In a further example, FIG. 7C depicts secondary gearbox 300 with bypassassembly 132 in the engaged position. In the illustrated embodiment,multimode clutch assembly 128 includes one or more oil pressure sensors504 which may be positioned within actuator liner 304 or may beotherwise located within the secondary gearbox downstream of an oilpressure passageway. Oil pressure sensors 504 are selectively alignedwith one or more ports 506 of piston 306 that are in communication withthe lubrication circuit of secondary gearbox 300 to detect the presenceor absence of oil pressure and/or a high pressure or low pressurecondition. Specifically, when bypass assembly 132 is in the disengagedposition, ports 506 are not aligned with oil pressure sensors 504whereas, when bypass assembly 132 is in the engaged position, ports 506are aligned with oil pressure sensors 504. A pressure reading below apredetermined threshold by oil pressure sensors 504 indicates bypassassembly 132 is not in the engaged position while a pressure readingabove a predetermined threshold indicates bypass assembly 132 is in theengaged position, thereby providing the engagement status of bypassassembly 132.

FIG. 7D depicts secondary gearbox 300 with bypass assembly 132 in theengaged position. In the illustrated embodiment, multimode clutchassembly 128 includes an engagement status sensor depicted as a variabledifferential transformer in the form of a linear variable differentialtransformer 508. Linear variable differential transformer 508 includes acore 508 a that is coupled to the end of oil jet 146 that translateswithin a coil assembly 508 b such that an input voltage within coilassembly 508 b induces two output voltages as piston 306 is shiftedbetween first and second positions. The electrical signals generated inresponsive to the rectilinear motion of core 508 a relative to coilassembly 508 b is used to determine the engagement status of bypassassembly 132.

FIG. 7E depicts secondary gearbox 300 with bypass assembly 132 in theengaged position. In the illustrated embodiment, multimode clutchassembly 128 includes an engagement status sensor depicted as a variabledifferential transformer in the form of a rotary variable differentialtransformer 510. An input shaft of rotary variable differentialtransformer 510 is rotated by gear teeth 512 on piston 306 as piston 306is shifted between first and second positions. Rotary variabledifferential transformer 510 provides a variable alternating currentoutput voltage that is linearly proportional to the angular displacementof the input shaft. These electrical signals are used to determine theengagement status of bypass assembly 132.

The foregoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principalsof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Other substitutions, modifications, changes and omissions may be made inthe design, operating conditions and arrangement of the embodimentswithout departing from the scope of the present disclosure. Suchmodifications and combinations of the illustrative embodiments as wellas other embodiments will be apparent to persons skilled in the art uponreference to the description. It is, therefore, intended that theappended claims encompass any such modifications or embodiments.

What is claimed is:
 1. A powertrain for a rotorcraft, the powertraincomprising: a main drive system including a main engine; a secondaryengine; and a failsafe multimode clutch assembly positioned between themain drive system and the secondary engine, the clutch assemblyincluding: a freewheeling unit having an input race coupled to the maindrive system and an output race coupled to the secondary engine, thefreewheeling unit having a driving mode in which torque applied to theinput race is transferred to the output race and an overrunning mode inwhich torque applied to the output race is not transferred to the inputrace; a bypass assembly having an engaged position in which the bypassassembly couples the input and output races of the freewheeling unit anda disengaged position in which the bypass assembly does not couple theinput and output races of the freewheeling unit; and an actuatorassembly having a default configuration in which a pressurizedlubricating oil provides an engagement force that maintains the bypassassembly in the engaged position and an energized configuration in whicha disengagement element provides a disengagement force that overcomesthe engagement force and shifts the bypass assembly from the engagedposition to the disengaged position; wherein, in the disengaged positionof the bypass assembly, the overrunning mode of the freewheeling unit isenabled such that the clutch assembly is configured for unidirectionaltorque transfer from the input race to the output race; and wherein, inthe engaged position of the bypass assembly, the overrunning mode of thefreewheeling unit is disabled such that the clutch assembly isconfigured for bidirectional torque transfer between the input andoutput races.
 2. The powertrain as recited in claim 1 wherein the mainengine further comprises a first gas turbine engine and wherein thesecondary engine further comprises a second gas turbine engine.
 3. Thepowertrain as recited in claim 1 wherein the main engine furthercomprises a gas turbine engine and wherein the secondary engine furthercomprises an electric motor.
 4. The powertrain as recited in claim 1wherein the secondary engine is configured to generate between about 5percent and about 20 percent of the power of the main engine.
 5. Thepowertrain as recited in claim 1 wherein the secondary engine isconfigured to generate between about 10 percent and about 15 percent ofthe power of the main engine.
 6. The powertrain as recited in claim 1wherein the engagement force of the pressurized lubricating oil isconfigured to shift the bypass assembly from the disengaged position tothe engaged position when the disengagement force of the disengagementelement is not provided.
 7. The powertrain as recited in claim 6 whereinthe disengagement element further comprises a pressure switch.
 8. Thepowertrain as recited in claim 7 wherein the pressure switch furthercomprises a hydraulic switch.
 9. The powertrain as recited in claim 7wherein the pressure switch further comprises a compressed air switch.10. The powertrain as recited in claim 6 wherein the disengagementelement further comprises an electric switch.
 11. The powertrain asrecited in claim 6 wherein the default configuration of the actuatorassembly further comprises an unenergized state of the disengagementelement.
 12. The powertrain as recited in claim 1 wherein the actuatorassembly further comprises a liner and a piston, the piston slidablydisposed relative to the liner and coupled to the bypass assembly; andwherein, the pressurized lubricating oil acts on differential areas ofthe piston to bias the bypass assembly toward the engaged position. 13.The powertrain as recited in claim 12 wherein the liner and the pistondefine an oil chamber therebetween; and wherein, the differential areasfurther comprise annular differential areas of the piston formingopposite sides of the oil chamber.
 14. The powertrain as recited inclaim 13 wherein the pressurized lubricating oil flows through the oilchamber.
 15. A rotorcraft comprising: a main rotor coupled to a maindrive system including a main engine; a secondary engine; and a failsafemultimode clutch assembly positioned between the main drive system andthe secondary engine, the clutch assembly including: a freewheeling unithaving an input race coupled to the main drive system and an output racecoupled to the secondary engine, the freewheeling unit having a drivingmode in which torque applied to the input race is transferred to theoutput race and an overrunning mode in which torque applied to theoutput race is not transferred to the input race; a bypass assemblyhaving an engaged position in which the bypass assembly couples theinput and output races of the freewheeling unit and a disengagedposition in which the bypass assembly does not couple the input andoutput races of the freewheeling unit; and an actuator assembly having adefault configuration in which a pressurized lubricating oil provides anengagement force that maintains the bypass assembly in the engagedposition and an energized configuration in which a disengagement elementprovides a disengagement force that overcomes the engagement force andshifts the bypass assembly from the engaged position to the disengagedposition; wherein, in the disengaged position of the bypass assembly,the overrunning mode of the freewheeling unit is enabled such that theclutch assembly is configured for unidirectional torque transfer fromthe input race to the output race; and wherein, in the engaged positionof the bypass assembly, the overrunning mode of the freewheeling unit isdisabled such that the clutch assembly is configured for bidirectionaltorque transfer between the input and output races.
 16. The rotorcraftas recited in claim 15 wherein, in a preflight configuration, the bypassassembly is in the disengaged position, the main engine is not operatingand the secondary engine provides power to at least one rotorcraftaccessory.
 17. The rotorcraft as recited in claim 15 wherein, in anenhanced power configuration, the bypass assembly is in the engagedposition, the main engine provides power to the main drive system andthe secondary engine provides power to at least one rotorcraft accessoryand to the main drive system through the clutch assembly.
 18. Therotorcraft as recited in claim 15 wherein, in a high efficiencyconfiguration, the bypass assembly is in the engaged position, thesecondary engine is in standby mode and the main engine provides powerto the main drive system and to at least one rotorcraft accessorythrough the clutch assembly.
 19. The rotorcraft as recited in claim 15wherein, in an enhanced autorotation configuration, the bypass assemblyis in the engaged position, the main engine is not operating and thesecondary engine provides power to the main drive system through theclutch assembly.